Scleractinian coral morphogenesis searching for stem cells and precursors of the skeletogenic calicodermis

Metamorphosis is a crucial step in the life cycle of a scleractinian coral: The swimming coral larva, the planula, settles, and metamorphoses into a sessile calcifying primary polyp, which subsequently grows into an adult colony. Importantly, morphogenesis into a primary polyp involves the establishment of the skeletogenic cell layer, the so-called calicodermis, but the cellular mechanisms that support this process are not well understood. In the work presented here, calicodermis establishment was studied in two scleractinian corals, Pocillopora damicornis and Stylophora pistillata using cell proliferation and apoptosis assays, electron microscopy, molecular biology tools, and isotopic imaging using NanoSIMS. Cell proliferation during coral metamorphosis was evaluated with BrdU (5-bromo-2â-deoxyuridine) incuba-tion pulses (24 h). In the primary polyp stage, cell migration was observed in the chase phase for several days (48 to 64 h) after the incubation pulse. A number of key observations were made: 1) Right after settlement, during the earliest metamorphosis stage that includes initial skeletal formation and therefore requires a fully functional calicodermis, cell proliferation rate in this tissue layer is in fact not significantly higher than in the preceding planula larval stage. This indicates that calicodermis establishment is primarily happening through transdifferentiation of cells from the aboral pseudostratified epithelium of the planula into calicodermis cells. 2) Later, in the growing primary polyp local cell proliferation in the calicodermis remains low, indicating that these cells do not originate in the calicodermis itself. 3) At the same time, it is observed in S. pistillata that the pharynx is the most proliferative area with up to 19% of cells dividing, and that cells migrate from this area through pseudostratified epithelium and/or mesenterial filaments to reach the expanding calicodermis, which is actively forming skeleton at this stage. 4) In order to identify potential stem cells and calicodermis precursor cells locations, biological markers where developed. Potential stem cells expressing piwi genes were localized in the mesenteries and below tentacle tips of P. damicornis. These sites contain proliferative areas involved in gametogenesis and nematogenesis, respectively. 5) Pdcyst-rich is a protein localizedin the adult calicodermis and potentially involved in skeleton formation. The related gene was observed to be expressed at the early metamorphosis stage, when skeleton deposition commences. Preliminary results indicate that Pdcyst-rich proteins are also located in the calicodermis of the forming polyp and skeleton of the primary polyp stage. This suggests a role of Pdcyst-rich in skeletogenesis but it is not clear if it acts as a protein of the skeletal organic matrix or as an adhesion protein.